The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Automatic transmissions use a fluid clutch known as a torque converter to transfer engine torque from the engine to the transmission. The torque converter operates through hydraulic force provided by pressurized fluid from the automatic transmission. The torque converter multiplies engine torque and directs it through the transmission.
A conventional torque converter includes a sealed chamber filled with hydraulic fluid. The chamber includes a pump (or impeller) driven by the engine, a turbine connected to an output shaft, and a stator that provides torque multiplication. As the impeller rotates, the centrifugal force pushes the pressurized fluid outward, causing the turbine to rotate. Fluid exiting the turbine strikes the stator. Blades of the stator act to reverse the radial direction of the fluid's motion so that the fluid is moving the same direction as the impeller when it reenters the impeller chambers. This reversal of direction greatly increases the efficiency of the impeller. The force of the fluid striking the stator blades also exerts torque on the turbine output shaft, providing additional torque multiplication equivalent to a higher numerical gear ratio.
A torque converter is said to “slip” when the impeller speed and the turbine speed are not equivalent. High slip rates reduce the torque converters efficiency and may generate excessive heat. Some converters incorporate a lockup mechanism such as a mechanical clutch that engages at cruising speeds to physically link the impeller with the turbine. The physical link causes the impeller and the turbine to rotate at the same or near the same speed, thereby reducing or eliminating slip. The clutch is applied and released via fluid supplied through a hollow shaft at the center axis of the rotating converter assembly.
Locking the torque converter clutch is not desirable in all modes of vehicle operation. Typically the torque converter clutch can only be fully locked during high speed, low throttle (cruising) conditions. To achieve the benefits of locking the torque converter clutch during other modes of operation, a material can be added to the clutch to improve its overall durability during slip conditions. This allows the clutch to be partially locked. Slip on these types of torque converters is electronically controlled to be near a target value (most commonly 20 rpm).
The degree of engaging the torque converter clutch may be regulated by commanding the torque converter to operate in one of a plurality of modes. When an “on mode” is commanded, pressure to the clutch is electronically controlled to achieve the target slip value. The converter is not completely locked. When a “lock on mode” is commanded, a maximum pressure is supplied to the torque converter to fully lock the torque converter clutch. Slip is essentially eliminated in the lock on mode. Throughout the drive cycle, torque converter operation transitions between these and other modes. Regulating the pressure supplied to the torque converter during these transitions improves the overall drivability of the vehicle.